class_docstr: A stream operator for aggregating an entire history of a stream.
  
  Parameters
  ----------
  func : `Callable[[List[float]], float]`
      A function that aggregates the history of a stream.
code: def forward(self) -> float:
      expanding = self.inputs[0]
      history = expanding.value
      if len(history) < expanding.min_periods:
          return np.nan
      return self.func(history)
  
imports: from typing import Callable, List
  import numpy
  from tensortrade.feed.core.base import Stream
  from tensortrade.feed.api.float import Float
label_desc: Use image node_id 2 for calling the ExpandingNode obj's underlying member method code with example usage: obj.forward() and returns: self, np